Polarization mode dispersion (hereinafter, “PMD”) is an optical property that can be generated by a concatenation of two or more birefringent elements. PMD can be a significant impairment in high data-rate optical communication systems when the transmission medium is optical fiber. Data transmission rates that are effected by the PMD of optical fiber are typically 10 Gbps, 40 Gbps, and higher.
Optical fiber can exhibit PMD because of imperfections within the fiber, which induce localized birefringence. When the transmission path is long, these localized birefringent sections can combine to yield a particularly complicated polarization-dependent effect. These localized sections are known to result, for example, from eccentricities of the waveguide's core, micro-bubbles in the waveguide core and/or cladding, and strain gradients through the fiber cross-section. Mechanical stress on the fiber resulting from cabling and installation can also cause the fiber to suffer stress-induced birefringence. Environmental changes experienced by a fiber can be dynamic and statistical in nature, and are believed to result in PMD changes that can last for variable periods of time and vary with wavelength, with the potential for prolonged degradation of data transmission.
In the laboratory and the field, there are reasons to artificially generate PMD in a controlled fashion.
In the laboratory, for example, a PMD emulator is desirably used to predictably and repeatably add PMD to signals generated by optical transmitters for testing optical receivers. In many cases, however, the center frequency of the optical signal being tested may not be properly aligned with the PMD spectrum generated by the emulator. Because a conventional PMD emulator cannot controllably “frequency shift” its spectrum to accommodate for the misalignment, those attempting to evaluate the PMD response of receivers and other equipment are generally forced to test undesirable and unpredictable PMD states. Often, PMD emulators include ten or more birefringent sections.
A PMD generator can also be incorporated into a specialized telecommunications sub-system called a PMD compensator. PMD compensators are used to mitigate the deleterious effects of PMD imparted on an optical data signal transmitted through an optical fiber. In contrast to PMD emulators, PMD compensators generally include only one or two birefringent sections, but such a small number of sections greatly limits the range of achievable PMD states. In order to achieve a greater operating range, it may be desirable to use PMD compensators that include more than two birefringent generator sections. Unfortunately, PMD spectra generated with more than two sections are difficult to control, subject to misalignment, and are typically frequency dependent.
In some cases, PMD can deleteriously reshape propagating optical pulses, and the degree and type of reshaping can depend on the type of PMD impairment. Generally, impairment includes two such types: first order PMD and second order PMD.
First order PMD is commonly referred to as differential group delay (hereinafter, “DGD”), and more particularly, as the DGD at a small frequency bandwidth. Pure first order PMD can be generated by a single homogenous birefringent medium.
Second order PMD has two parts: polarization-dependent chromatic dispersion (hereinafter, “PDCD”) and depolarization. PDCD is the mathematical derivative of DGD with respect to frequency. Depolarization relates to a change of the Stokes PMD parameters with frequency.
Pure first and second order PMD (i.e., the case in which the second-order PMD only includes depolarization) can be generated using a concatenation of two birefringent sections. Higher orders of PMD can introduce curvature of the DGD spectrum, and complicated contortions of the Stokes PMD parameters, with respect to frequency. In contrast to the generation of pure first and second order PMD, which only uses two birefringent sections, higher orders of PMD can be generated using three or more birefringent sections.
DGD, depolarization, PDCD, and other higher orders of PMD can impair optical data transmission in characteristic ways. To analyze the degree to which PMD impairs an optical signal, predictable and repeatable generation of the individual PMD components is desirable. Thus, as mentioned above, PDCD and higher order PMD components can be generated using three or more birefringent stages.
A PMD generator that can predictably access the plurality of PMD components may still not possess maximum utility. PDCD and higher PMD orders can exhibit frequency dependence. As such, a spectrum, for example a PDCD spectrum, has a shape determined by the particular construction and settings of the generator. At some frequencies the spectrum may be at a minimum or maximum, and at other frequencies the spectrum may change quickly. Some PMD generators can produce PMD spectra that are substantially periodic with optical frequency, regardless of the particular spectral shape. Generally, the period of a spectrum is larger than the bandwidth of an optical data signal.
In order to more completely measure the PMD impairment of an optical data signal, the signal should experience all segments of an artificially generated PMD spectrum. One method to measure signal impairment across all segments of an artificially generated PMD spectrum is to tune the frequency of the optical signal. Another method to measure signal impairment across all segments of an artificially generated PMD spectrum is to shift in frequency the PMD spectrum while maintaining the shape of the PMD spectrum intact. Generally, both methods can produce similar results, but the more useful method depends on the experimental setup.
It would therefore be desirable to provide methods and apparatus for generating PMD, and in particular DGD, depolarization, PDCD, and higher order PMD in a controllable, predictable, and reliable way.
It would also be desirable to provide methods and apparatus that are capable of frequency shifting a PMD spectrum while preserving its shape.
It would also be desirable to provide methods and apparatus for compensating PMD impairment.
It would also be desirable to provide methods and apparatus for generating PMD for PMD characterization purposes.